United States' liquid rocket propulsion technology has evolved over the past five decades to meet the changing needs of the commercial community, agree with Department of Defense (DoD) diminishing budgets, and ensure national access to space.[3] The importance of the United States having assured access to space is best laid out by the National Presidential Directive Number 40 which states:

Access to space through U.S. space transportation capabilities is essential to:

place critical United States Government assets and capabilities into space;

augment space-based capabilities in a timely manner in the event of increased operational needs or minimize disruptions due to on-orbit satellite failures, launch failures, or deliberate actions against U.S. space assets;

support government and commercial human space flight.

The United States, therefore, must maintain robust, responsive, and resilient U.S. space transportation capabilities to assure access to space.[4]

The United States’ current solution to assure space access for operational space assets is to maintain two families of launch vehicles under the EELV program. The EELV program was initiated in 1995 as the Air Force’s premium space lift modernization program. The purpose of this program was to reduce the cost of operational space launch by 25–50% and to improve reliability over the heritage launch systems (Atlas II, Delta II, and Titan IV). Procurement of EELV boosters for military space launch was to evolve into a "commercial like" nature.[5] The EELV program eventually produced two families of launch vehicles as the solution to US space lift needs. These two families are the Delta IV launch system, developed by McDonnell Douglas (now The Boeing Company), and the Atlas V launch system, developed by Lockheed Martin.

The Atlas V and Delta IV Space Launch Boosters Make Up EELV. The United States Air Force (USAF) oversaw the development of the two new launch systems in just five years.[5] Boeing’s Delta IV and Lockheed Martin’s Atlas V programs eventually merged to form the United Launch Alliance (ULA).

In April 2016, SpaceX won a contract for its Falcon 9 rocket under the EELV program to launch a GPS III payload to MEO.[6]

The Atlas V and Delta IV systems evolved through different paths. Major differences include launch processing, launch pad operational concept, and some major components. Falcon 9 was developed privately by SpaceX.

The Atlas V space launch system has a lineage which began in 1954 under the Intercontinental Ballistic Missile (ICBM) program.[7] The first US Intercontinental Ballistic Missile (ICBM) was the SM-65 Atlas. Although the Atlas started as ICBM technology, emergence as a space launch vehicle (SLV) followed soon thereafter.[7]

Main features include the Common Core Booster powered by a RD AMROSS RD-180 engine, Aerojet strap-on solid boosters (up to five), Centaur (rocket stage), powered by a single or dual Pratt & Whitney Rocketdyne RL10A-4-2 engine(s), and an option of a 4.2 or 5.4 meter payload fairing (PLF). A three-digit (XYZ) naming convention is used for the Atlas V configuration identification. The first digit represents the payload fairing size (either 4 or 5), the second digit represents the number of solid rocket boosters used (0 through 5), and the third digit represents the number of engines used on the Centaur (1 or 2)

Main features include the Common Booster Core (CBC) powered by a Pratt and Whitney Rocketdyne RS-68 engine, a Delta Cryogenic Second Stage (DCSS), and off pad horizontal vehicle integration. The Delta IV M configuration consists of a CBC first stage and a 4-m diameter DCSS.[8] There are three variants of Delta IV M+ configuration. The Delta IV M+(4,2) uses two strap-on solid rocket motors (SRMs) solid rocket booster to augment the first-stage CBC and a 4-m diameter DCSS and PLF. The Delta IV M+(5,2) and Delta IV M+(5,4) have two and four SRMs, respectively and 5-m diameter DCSS and PLF. The Heavy Lift Vehicle (HLV) variant has two strap-on CBC cores with a 5-m DCSS and PLF. The Delta IV-Heavy utilizes the same configuration as the Delta IV-M (no strap on Aerojet boosters) but with two additional Common Booster Cores.

The U.S. Air Force assembled its initial blue print for the EELV in 1994, following many years of government funded studies into an improved system and architecture, which was intended to replace most if not all existing "legacy" spacelifters (e.g. Delta II, Atlas II/Centaur, Titan IV, etc.). The architecture called for the spacelifter to be based on standardized fairings, liquid core vehicles, upper stages, and solid rockets. The Standard Payload Interface bus was also proposed as another way to save money and improve efficiency.

Boeing and Lockheed Martin were both collectively awarded US$100 million for the final phase of the bid. Both companies built their designs around modularization, standardization and minimizing the amount of equipment and using proven, reliable and simplified systems. Boeing developed the Common Booster Core (CBC) that would be the center of the Delta IV. For the Atlas V Lockheed Martin did something similar, calling it a Common Core Booster (CCB).[10]

Boeing was found to be in possession of proprietary documents from Lockheed Martin.[11] To end litigation, both companies agreed to join forces and formed the United Launch Alliance joint venture.[12] Each company has a 50% stake in ULA.[13]

In October 1998 two initial launch services contracts (known as Buy 1) were awarded. Along with the award of two development agreements, the total amount was more than $3 billion.[10] Boeing was awarded a contract for 19 out of the 28 launches; Lockheed Martin was awarded a contract for the other 9. Boeing received $1.38 billion, and Lockheed Martin received $650 million for the launches.[14] In 2003 the USAF moved 7 launches from Delta IV to Atlas V.[15]

In December 2012, the DoD announced a re-opening of the EELV-class launch vehicle market to competition beginning in 2015. "Under the new plan, the Air Force is authorized to proceed with a block buy of "up to" 36 launch cores from current monopoly vendor United Launch Alliance, while at the same time opening up another 14 cores to be purchased competitively. The new era will begin in 2015 with initial launches to be performed in 2017."[16] The Air Force signed a contract at that time with SpaceX for two launches in 2014 and 2015 to serve as proving flights to support the certification process for the Falcon 9 v1.1 and Falcon Heavy.[17]

Following delays, the Air Force certified the Falcon 9 in May 2015.[18] Meanwhile, SpaceX and the Air Force settled SpaceX's lawsuit contesting launches awarded noncompetitively to ULA by opening up more launches to competitive bidding.[19]

For the launch vehicle for the Orion spacecraft, the Aerospace Corporation was asked by NASA three times, in 2005, 2008 and 2009, to assess technical feasibility and cost of modifying an EELV to be human-rated for use in NASA human spaceflight missions.[20] Two later assessments also addressed the possibility of replacing the Ares I with a Delta IV Heavy. The reports indicate that the Delta IV Heavy meets ISS and lunar target performance requirements.[21] Unlike other modifications of the Delta IV, the Heavy variant does not use solid rocket boosters.

In a presentation to the Review of U.S. Human Space Flight Plans Committee, an Aerospace Corporation representative presented a summary of the study. The summary asserts that even without a newly developed upper stage, it would be feasible for a human-rated Delta IV Heavy to launch a crewed Orion spacecraft to the International Space Station.[22] ULA has since published a paper[23] detailing the changes needed for man-rating EELVs. On February 2, 2010 NASA awarded[24] ULA $6.7 million in stimulus funds under the Commercial Crew Development (CCDev) program. A Space Act Agreement was set up to develop an Emergency Detection System (EDS) that could be used on both EELVs. An EDS monitors critical launch vehicle and spacecraft systems and issues status, warning and abort commands to the crew during their mission to low Earth orbit.